The Trimesters of Pregnancy

Written by

Rob Buist

Published on


A very long time ago somebody figured out that a normal human pregnancy lasted nine lunar months. This link – incredibly – was not always known. I’m told the ancient Egyptians were unaware of the link between the menstrual cycle and pregnancy. Accordingly they used to tie a reed gently around the necks of young brides. When the reed broke – because of swelling of the thyroid gland – they recognised the woman was pregnant. 

Anyway eventually somebody figured out the link between pregnancy and the months and, well, nine divides into three easily so the trimesters were recognised. It’s worth remembering that the trimesters are a numerical construct and accordingly shouldn’t be taken too seriously. Traditionally though we have ascribed a number of features, or symptoms, to each trimester:

The First Trimester

Obviously the first trimester follows conception and – in many respects – is not the easiest trimester. Nausea and vomiting can be a significant factor in the first trimester but unfortunately that is not all. Most women – even if they don’t suffer from nausea and vomiting – feel exhausted throughout the first trimester and in addition shortness of breath and heart palpitations can be problematic. These symptoms reflect the MASSIVE physiological changes occurring in your body from around 8 week’s gestation onwards. Even if women are not nauseated they find themselves gravitating towards bland “comfort” food – often carbs. 

All of the baby’s organs become fully formed during the first trimester.

So all of this can mean the first trimester can be quite difficult (and of course many women may not have told their friends, family and colleagues their big news so feeling awful can be even more difficult). I often tell women that the first trimester is to be survived – yes we know you’re supposed to eat healthy food and exercise lots in pregnancy but most women need to leave such healthy living until the second trimester.

Medical complications that can arise in the first trimester include miscarriage and ectopic (tubal) pregnancy. The first trimester is also when we organise your tests for the baby suffering from a chromosomal abnormality such as Down syndrome and most women have chosen their pregnancy caregiver and the place they intend to give birth by the end of the first trimester.

The Second Trimester

If women really do bloom during their pregnancies they do so during the second trimester. This time – from around 13 to around 27 weeks gestation – is a period of significant growth of the baby. During this time the baby grows from a tiny fetus into a baby weighing one kilogram or so that can – if necessary – survive outside of the womb (thanks to advances in newborn intensive care).

Most women do feel very well during their second trimester and this is a time when they can get on with eating healthily and exercising as much as possible. Most women begin to feel their baby moving inside them at around 19 weeks (first pregnancy) or 16 weeks (second and subsequent pregnancies) although this timeline can depend on the location of your placenta – a so called anterior placenta (i.e. one in front of your baby) – can “muffle” your perception of movements such that you may not feel movement until after 20 weeks.

The second trimester is the time when you need to organise your labour and birthing classes (your care provider will point you in the right direction). In Australia we perform blood tests on pregnant women testing for low iron levels and Gestational Diabetes (GDM) at around 26 to 28 weeks gestation.

The medical pregnancy complications we watch out for in the second trimester (apart from GDM) include early preeclampsia or PET (high blood pressure in pregnancy), fetal growth restriction and premature labour or birth. Fortunately these complications are very rare in the second trimester.

The Third Trimester

The third trimester lasts from 27 weeks gestation until your baby is born (at usually around 40 week’s gestation). Initially at least the third trimester is much like the second trimester – most women notice their baby growing but they usually feel very well and able to work, exercise and do all of the things they wish to do. Unfortunately the increasing size of your baby may take its toll as the third trimester of pregnancy progresses. Some women begin to experience not so pleasant symptoms like interrupted sleep, heartburn, varicose veins, haemorrhoids and pelvic discomfort. Energy levels can fall and you can find yourself having “Nana Naps” when you get the chance. General lowering energy levels (not to mention the possibility of going into labour) mean that most women stop working and go onto maternity leave somewhere around 36 to 38 weeks gestation.

From a medical perspective our main tasks during the third trimester are watching your blood pressure (in case preeclampsia arises) and checking that your baby is growing appropriately. Your caregiver will ask you to keep an eye on your baby’s movements throughout the third trimester and expect you to seek help if you feel your baby is significantly less active than usual.

There is one other thing to think about during the third trimester – the birth itself! Most women – even if they don’t have a formal “Birth Plan” – will have thought about the birth and their preferences for that event and their “hospital” bag will be packed and ready to go. They will have spent a fortune on prams, cots, change tables and all of the paraphernalia that goes with the arrival of their newborn.